Parameterized AttributeConverter and/or AttributeConverter metadata access

It would be good if we were able to parameterize AttributeConverter instances and/or have access to property metadata within an AttributeConverter. This way we are able to reuse AttributeConverter code instead of building similar ones or even supply further required context information. An example:

Quite often we have to deal with ancient DB schemas, and just recently we started to migrate a large Cobol application to Java. We are unable to change the content of the database, and because of Cobol with have to support fixed width string columns containing left padded data. A 5 digit fixed width column will represent 1 as '00001' and 100 as '00100'. Nothing we can change here. Unless I am mistaken I have to create a dedicated converter for every required fixed length ending up with an LeadingZeroTwoDigitAttributeConverter, LeadingZeroFiveDigitAttributeConverter and so on.

Right now it seems the JPA spec does not define wether there is one converter instance per persistent property or just a global one (which would be OK by current spec requirements). I would propose that parameterized converter instances are bound to property fields in order to support parameter evaluation during JPA provider startup.

I believe we need access to both basic attribute information and optionally supplied converter attributes. The following example would assume access to the leading pad char, the length attribute and wether the attribute is nullable or not. (in my current project I have to store an empty string for null values in the database).

@Convert(converter = LeadingDigitAttributeConverter.class, metaData="padChar=0" )
@Column(name = "ACOLUM", length=5, nullable = false)

If you believe there is no need to support something like that: Unfortunately we are in the middle of migrating quite a few very very old applications to Java, and we can't change that stuff. And there is more to come.

Start by reading the AttributeConverter, @Convert, and @Column entry points in the current JPA specification and compare them with the proposal in issue #73. Determine the required metadata and parameterization contract before implementation; done would require an agreed specification design, provider behavior, and corresponding tests.

Written by the indexing model from the issue text.